The present invention relates to devices for coupling together two circuit components having different impedances, particularly in systems in which power is to be supplied from a power source to a component which has an impedance that varies during operation.
A basic technique already known in the art is to provide a conical, or tapered, transmission line having an impedance which varies along its length between two components having different impedances. The transmission line constitutes a fixed impedance transformer generally considered suitable for connection between two elements whose impedances do not vary with time.
Commonly used RF power sources, or generators, are designed to operate at 50 ohms output impedance. Regardless of the value which the output impedance may have, if a device with an input impedance other than the power source output impedance needs to be powered, a match network is used to match the device input impedance with the output impedance of the RF power generators. A device""s input impedance can have real and/or complex components. In general, when the output impedance of a source is equal to the input impedance of a load, the transfer of power is most efficient.
Such RF power generators are used to drive electrodes which couple electromagnetic energy into a plasma chamber employed to perform etching and deposition operations in connection with, for example, semiconductor wafer fabrication. The electrodes are frequently referred to as plasma electrodes. In such apparatus, it is common practice to connect a match network with a connecting element in the form of a RF feed between the power generator and the plasma electrode in order to match the plasma system input impedance with the output impedance of the RF power generator.
The plasma system input impedance, however, is complex in nature and varies greatly with chamber conditions, being determined by many factors including the chamber pressure and geometry, species of gas, power, and discharge plasma impedance. It is useful to consider the plasma impedance as a range of impedances that varies with the operating conditions of the plasma. In general, the plasma impedance associated with the fundamental frequency is higher than the plasma impedances associated with high frequency harmonics. As the driving frequency on the electrodes increases, the plasma density increases and the high frequency harmonics also increase.
The biggest change in the plasma system impedance is between the plasma start condition and the plasma lit, or run, condition. In the plasma start condition, there is no plasma so the discharge plasma impedance is high. Therefore, the plasma system input impedance is the impedance determined by the system hardware alone. In the plasma lit condition, the discharge plasma impedance is complex and varies with time. The plasma system input impedance is therefore a complex impedance determined by the system hardware and the discharge plasma impedance. The match network therefore must be capable of adjusting to the load impedance changes over time and between the plasma start condition and the plasma lit condition, as well as the more subtle variations in impedance associated with changing plasma conditions, without extinguishing the plasma.
The demands placed on the match networks used for driving the electrodes of a plasma chamber are further complicated by the effect of protection circuitry in the RF power generator. This circuitry is designed to reduce the power output when the power reflected back to the power generator is higher than a certain value and is usually limited to the fundamental frequencies, or low frequencies, only because most of the power that can cause damage is reflected in those frequencies. However, the power contained in higher frequency harmonics in the RF power may reflect back through the protection circuit as a RF load to destroy the RF power circuits of the power generator. Thus, the match networks further require a capability of matching the fundamental frequencies as well as the higher frequencies of the harmonics. As the driving frequency applied to the plasma electrodes increases, the plasma density increases and the plasma impedance decreases until the plasma becomes predominantly inductive, and the high frequency harmonic contents and the power in these harmonics increase, the match networks need to match an even broader impedance range.
Recent development trends in plasma chamber design include the use of higher plasma excitation frequencies. The higher frequencies produce, in addition to a higher fundamental frequency, a higher plasma density and a higher plasma harmonics content. This will result in a lower plasma impedance associated with the higher fundamental frequency and a lower plasma impedance associated with each higher plasma harmonic as well. The net effect is that the input impedance range for the fundamental frequency for the plasma chamber system is shifted to a lower level, but extends over a wider range.
According to conventional practice in the art, the connecting RF feed is short compared to the wavelengths associated with the fundamental frequency it delivers and is made of components which provide a low loss at the fundamental frequency. This ensures that the transmission loss is minimized. It is possible to dissipate 20-30% of the fundamental RF power by heating this connecting RF feed.
Another problem presented by existing RF feeds is that refitting errors can occur when the drive system is disassembled and then the RF feed is connected to the electrode during reassembly. This refitting error in general will not affect transfer of energy at the fundamental frequency, but has enormous impact on how the various harmonics are handled in the system.
The invention provides an AC power feed device for conducting electrical power from a source to a load, the device having an input end constructed to be coupled to the source, an output end constructed to be coupled to the load and a length dimension between the input end and the output end. The feed device has a constant impedance along the length dimension and an outer periphery that varies in size progressively along the length dimension.
The invention further provides a system for supplying high frequency power to a load having an impedance which varies with time, the system being composed of: an electrode having a characteristic impedance disposed to couple the power into the load and providing, with the load, a load impedance; a match network for supplying the power to the electrode, the match network having an output impedance value which varies with time as a function of variations in the load impedance value; and a feed device connected for conducting the power from the match network to the electrode. The feed device has an input end connected to the match network, an output end connected to the electrode, and a length dimension between the input end and the output end. The feed device further has an impedance which varies from a value at the input end substantially equal to the output impedance of the match network to a value at the output end substantially equal to the characteristic impedance of the electrode.
The characteristic input impedance of the feed line will be chosen to substantially equal the match network output impedance at xe2x80x9cnominalxe2x80x9d conditions. Nominal conditions would be process conditions that exist for most of the time during a typical process, determined by RF power, chamber pressure, etc.
Embodiments of the invention can be in the form of a tapered conical RF feed structure with constant impedance at any cross section. The tapered conical RF feed structure may have a constant impedance along its length and an electrical length, which is the coupling distance between the electrode and the match network, of x=nxcex/2, where xcex is the fundamental frequency wavelength.
Other embodiments can be in the form of a tapered conical RF feed structure an impedance which varies along its length. The impedance of the RF feed can be gradually increasing from an extremely low impedance near the electrode to a relatively high impedance near the match network port. In particular, the impedance of the tapered conical RF feed structure at one end matches the impedance of the electrode at a given frequency, and at the other end matches the impedance of the match network at the same given frequency.
More generally, a stripline can be connected between any two electrical components and the impedance at each end can be equal to that of the component connected to that end.
Even though a transmission line according to the invention can only approximately match the impedance at the upper edge of a plasma electrode, it nevertheless provides an improved coupling to the electrode
The invention can also be applied to reactor wafer chuck RF feeds.